modelFallbackMiddleware

import { createAgent, modelFallbackMiddleware } from "langchain";

// Create middleware with fallback models (not including primary)
const fallback = modelFallbackMiddleware(
  "openai:gpt-4o-mini",  // First fallback
  "anthropic:claude-sonnet-4-5-20250929",  // Second fallback
);

const agent = createAgent({
  model: "openai:gpt-4o",  // Primary model
  middleware: [fallback],
  tools: [],
});

// If gpt-4o fails, automatically tries gpt-4o-mini, then claude
const result = await agent.invoke({
  messages: [{ role: "user", content: "Hello" }]
});